Zeolites surface interactions

Table 2 Zeolite surface interactions investigated by dipolar NMR methods...

The tortuosity for pore-filling liquids is ideally a purely geometric factor but can, in principle, depend on the fluid-surface interaction and the molecular size if very small pores are present such as in zeolites (see Chapter 3.1). To obtain a measure for a realistic situation, we have used n-heptane as a typical liquid and have computed x... 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

Surface interactions between water and polymer networks have a profound effect on the water structure. The properties of water in these and other heterogeneous systems are sensitive to the size of the network pores and have been described by the two-phase model which assumes partition of the water between the "bulk and the "bound water phases" Evidence for this partition has been obtained in several proton NMR studies and also in ESR studies of paramagnetic probes in zeolites, silica gels and in water containing polymers. ... 

In conclusion, the EPR results demonstrate the presence of tetravalent vanadium atoms in some sites of the sample. The Vlv atoms could be included in a new compound, as in the NaV5vV,vC>15 vanadate identified in the sample by X-ray spectroscopy. They could also interact with the zeolite surface as reported for V205 supported on silica or n alumina (22) and titania surfaces (23). 

It has also been revealed [29] that the pore size and the affinity of a zeolite structure can be modified by chemical treatment of the zeolite structure the silane, borane, or disilane molecules are chemisorbed on the zeolite surface by reacting with the silanol groups of the zeolite [113], Polar molecules, for example, water and amines presorbed in the zeolite can be used to modify the operation of the molecular sieve and the interaction toward adsorbate molecules of the zeolite [113],... 

First, the use of Fe3(CO)] 2 and an extraction technique for the preparation of zeolite-supported iron catalysts results in the formation of highly-dispersed, small particle-sized Y F 2 3 on the zeolite surface with a small amount, 1 Wt % Fe, present in a nonoxide form that interacts strongly with, and may be incorporated into the pores of, the zeolite. 

Figure 1 An adsorption (top)-desorption (bottom) model for chiral induction on a zeolite surface, incorporating a reactant (tropolone alkyl ether, shown at the left), a chiral inductor (with four different substituents, at the right), and a cation (small ball on the surface). Tropolone s carbonyl and ether oxygens hydrogen-bond to chiral inductor, while its tt system interacts with zeolite s cation ion. ...

A more specific interaction between a zeolite surface and a chiral catalyst was recently uncovered (58). It was found that the Ru-binap catalyst can be specifically withheld on the outer surface of Beta zeolites. Such a heterogeneous catalyst is relevant for the highly enantioselective hydrogenation of methyl acetoacetate to (R) or (S) 3-hydroxymethylbutyrate, with typical ee values of about 95 %. 

Water has been widely used as a probe molecule for the characterization of zeolites, especially of those with a high aluminium content [9]. Water adsorption on hydrophilic zeolites has been used to measure their pore volume, and it has been shown that the amount of water adsorbed is a linear function of the aluminium content [10]. Additionally, water adsorption is also highly sensitive to the nature, valence and accessibility of extra-framework cations [11]. Immersion calorimetry allows for the measurement of the degree of interaction between the zeolite and water, and this can be compared with the interaction between the zeolite and other molecules with different polarity. In this way, the polar character of the zeolite surface can be assessed. 

The molecular field distribution within the channels must be investigated, taking into consideration the structure of the zeolite, and the calculation of the potential energy of interaction between the zeolite and particular molecules must be made. These investigations would be assisted greatly by spectroscopic studies which would make it possible to establish the nature of the zeolite surface, the presence and the nature of structural defects, and the state of the adsorbed molecules. 

The heats of adsorption of ethane and ethylene at zero coverage are presented in Table II. These heats are obtained by means of the virial equations (12). The heats of adsorption of ethylene exceed those of ethane because of specific interactions (7, 8). As the dimensions and polarizability of the ethane and ethylene molecules are similar, the energy of nonspecific interaction of these molecules with the zeolite surface must be approximately equal (6). Therefore, the differences of the heats of adsorption of ethane and ethylene, aQ, on the same zeolite is considered to be approximately the contribution of the specific interaction of the TT-bonds of ethylene with the corresponding cation. The values of these differences for all systems studied are presented also in Table II. 

Table VII presents a summary of calorimetric measurements of the differential heat of adsorption of ammonia, water, and carbon dioxide on the sodium form of ZSM-5 zeolite. Ammonia adsorption at 416 K (97.147) shows that NaZSM-5 zeolite is weakly acidic, whereas CO adsorption (147) indicates that in addition there are some weak basic sites. It should be noted that of the two samples studied with ammonia adsorption one was 70% H exchanged and the sodium content of the other was not given. Water adsorption on NaZSM-5 displayed unusual behavior, with a steep increase in the differential heat of adsorption at high surface coverages (166). An adsorption mechanism was proposed to explain these findings in which adsorption occurs first on the hydrophilic sites, consisting of sodium cations and framework anions where water molecules are bound by dipole-field interactions. Further adsorption takes place near these sites through weak interaction with zeolite surfaces, and when the number of water molecules close to these sites exceeds a certain value, they tend to reorient by forming clathrate-like struc-...

In view of the chemistry of this inert element, the main application of Xe NMR is as a surface probe for studying meso and microporous solids and the free volume in polymers. The relaxation time for Xe adsorbed in solids is typically 10 ms to a few seconds. The use of Xe NMR as a probe for studying microporous solids has been extensively reviewed by Barrie and Klinowski (1992). A more recent example of the use of Xe NMR to study surface interactions is provided by a study of borosilicalites with the ZSM-5 structure (Ngokoli-Kekele et al. 1998). The Xe shift of adsorbed xenon (referred to the shift of the pure gas extrapolated to zero pressure) was found to change regularly with boron content, with a discontinuity at a boron content of about one atom per unit cell ascribed to a change in the distribution of boron atoms in the lattice. A similar correlation between the Xe NMR shift and the aluminium content has been reported for the zeolite ZSM-5, in which the discontinuity occurred at about 2 Al atoms per unit cell (Chen et al. 1992). 

Since Cu ions on the zeolite surface exist in an isolated environment, they may interact with the sulfate species on the catalysts deactivated by SOj. The sulfate groups might partially surround the copper ions, as previously supested by Choi et al. [37] and Hamada et al. [38]. The sulfate may also have some characteristics of a coordinate covdent bond, where Cu ions and sulfate species may act as a Lewis acid and base, respectively [39], Ligands such as HjO, NHj, (C2Hj)3P, CO molecules and Cf, CN, OH , NOj, and C204 ions should at least contain a lone pair of electron to form a coordinate covalent bond between metal ions [40]. It should be noted that the sulfate catalyst species formed on the catalysts deactivated by SO2 contains lone electron pairs on O atoms which surround S atom of SO4 groups. Therefore, it is expected that the electrostatic interaction between Cu ions and sulfate species probably influences the local structure of Cu ions on the zeolite catalyst surface. 

Xenon adsorption isotherms are given in Figure 2. At low equilibrium pressure there is an increase in the amount of adsorbed xenon due to stronger interaction with the coke as compared to that with the zeolite surface. This effect is all the more important when the coke content is high. Conversely, at high equilibrium pressures, there is a decrease in the amount of adsorbed xenon due to a decrease in the pore volume by coke deposit and partial pore blocking. 

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